The expression "systematic thickness error" refers to the thickness errors which are determined by the system, for instance as a result of tool tolerances. They are unavoidable and occur without any intervention in the cross-sectional profile of the produced plastic foil, and in the installation for the production of the plastic foil always at the same locations. If no special steps are taken, these systematic thickness errors add up during the winding of the coil to positive or negative barrel-hoop formations in the roll of foil which is produced.
In the known processes which are the starting point of this invention, the attempt is made to avoid the summation of the thickness errors by displacement and statistical distribution of the thickness-error profile over the entire width of the plastic foil, so that during winding the summation of the thickness errors is replaced by an error distribution, so that the coil is practically free of the effects resulting from the local summation of thickness errors during coil winding.
Layer after layer, the thickness errors are in different locations in the coil. To achieve this in the production of so-called blown film, wherein at first a thermoplastic tube is produced, this tube is then blown to a tubular film and the tubular film is then flattened, either the blowing head with the tool for the tube extrusion must be given a periodic to and fro oscillating motion or the bubble or balloon flattening device must be correspondingly periodically moved. This requires an expensive construction and the results are often not fully satisfactory, because the resulting coil still has irregularities, even if it does not show pronounced barrel hoops.
In order to achieve the described error distribution when operating with a wide-slot nozzle it is known to reduce the magnitude of thickness errors, i.e. to level the thickness-error profile, according to a measured thickness-error profile, by adjusting the gap width of the wide-slot nozzle. The superposition and distribution of intended singular thickness errors is also known. As a result, these known steps the thickness tolerances can be reduced. However a disturbing thickness-error profile with a disturbing error addition are still be present in the coil.